The law is that they must REFLECT red. Tinting them isn't illegal because the lights aren't as bright (but could be a problem), they are illegal because after you tint them, they do not reflect red (when your lights are off).

If you look at the Lexus RX330 or RX400h or whatever, the tail lights are COMPLETELY white. But down on the bumper there are 2 big reflectors....

It looks nice, but I don't think it's worth getting pulled over for....

EDIT: Here are some pics of a RX400h.
http://img265.imageshack.us/img265/8084/400hiv2.jpg
http://img508.imageshack.us/img508/2737/400h2fy1.jpg

 

What they California DMV is implying is from a distance you perceive the tail light as a bright red light implying "stop". The bulb might be clear but the plastic is a dark red. The brightness level is important as they say it should be visible from 1000 feet distant. You can't really tint the plastic too much to drop lumen output considerably. Perhaps in the future you could have smoke red plastic combined with high output LED's that meet the DMV requirements.
Not much action online regarding tinted tail lights or shops offering to tint tail lights. The after market 35% smoke tint A3 tail lights specifically say "For offroad or show use only in most states" That kinda says it all! [sm=type.gif]